Systems, devices, and methods for designing and forming a surgical implant

ABSTRACT

A method is provided for determining the shape of a surgical linking device that is to be attached to a bony body structure such as the spinal column based on digitized locations of a plurality of attachment elements engaged to the bony structure. The method is implemented by a computer system through a GUI to generate an initial bend curve to mate with the plurality of attachment elements. The initial bend curve may be simplified based on user input to the GUI to reduce the number of bends necessary to produce a well-fitting linking device and may be altered to help obtain the goals of surgery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/898,271 filed on Aug. 29, 2022, which is a continuation of U.S.patent application Ser. No. 16/663,817 filed on Oct. 25, 2019 (now U.S.Pat. No. 11,453,041), which is a continuation of U.S. patent applicationSer. No. 15/496,628 (now U.S. Pat. No. 10,500,630) filed on Apr. 25,2017, which is a continuation of U.S. patent application Ser. No.14/049,183 (now U.S. Pat. No. 9,636,181), filed on Oct. 8, 2013, whichis a continuation of U.S. patent application Ser. No. 12/417,937 (nowU.S. Pat. No. 8,549,888), filed on Apr. 3, 2009, which is a continuationin part of U.S. patent application Ser. No. 12/246,581 (now U.S. Pat.No. 7,957,831), filed on Oct. 7, 2008, which is a continuation in partof U.S. patent application Ser. No. 12/098,375 (abandoned), filed onApr. 4, 2008, the entire contents of which are incorporated by referenceas if set forth in their entirety herein.

BACKGROUND

The present invention is directed systems, devices, and methods relatedto the design and formation of surgical implants such as surgicallinking devices. More particularly the present invention providessystems, devices, and methods for forming or shaping a surgical implantto conform to two or more selected attachment points (including surfaceanatomy) in a six degree of freedom method for attachment.

Fixation systems for aligning, adjusting and or fixing, either partiallyor rigidly, portions of a patient's bony anatomy in a desired spatialrelationship relative to each other are frequently used in orthopedicsurgery. For example, in spinal surgery for repair or positionaladjustment of the vertebrae, it is often necessary that multiplevertebrae are surgically manipulated. As spinal surgery often requiresthe instrumentation of more bony elements than other areas of orthopedicsurgery, the linkage devices can be extremely challenging to design andimplant. Treatment for conditions such as scoliosis, spinal injury, diskproblems and the like often make use of spinal rod fixation systems forpositioning the vertebrae and supporting the spinal motion segments.

A spinal rod needs to be oriented in six degrees of freedom tocompensate for the anatomical structure of the particular patient'sspine and the particular attachment points or methods for attaching therods to the vertebrae. In addition, the physiological problem beingtreated as well as physician's preferences will determine the exactconfiguration necessary. Accordingly, the size, length and particularbends of each spinal rod depends on the size, number and position ofeach vertebra to be constrained, their spatial relationship as well asthe fixating means, such as pedicle screws, used to hold the rodsattached to each vertebra. The relationship of the vertebrae will bedifferent for each patient and the positioning of the patient at thepoint of installation of the rods. During surgery, the orientation ofthe spine and vertebrae can be very different than the correspondingposition of a patient's upright posture. Rods are bent in one or moreanatomic planes measured by distance from each bend, angle of the bendand rotation in relationship to other bend points in order to fit intotwo or more vertebral anchors.

The bending of a spinal rod can be accomplished by a number of methods.The oldest and most widely used method for bending rods manually duringsurgery is a three-point bender called a French Bender in which abending pliers type device is manually operated to place one or morebends in a rod. The French Bender requires both hands to operate andprovides leverage based on the length of the handle. While the devicecan make it relatively easy to bend a spinal rod, the determination ofthe location, angle and rotation of bends using such a device is oftenarbitrary. Problems can thus occur from bending a device and thenrebending to fix mistakes which impose metal fatigue or stress risersinto a rod thus increasing the risk of a mechanical failure. Increasedtime in the operating room (OR) to achieve optimum bending of the rodcan increase the chance of morbidity.

Spinal rods are usually formed of stainless steel, titanium or othersimilarly hard metal, and as such are difficult to bend without somesort of leverage-based bender. In addition, since several spatialrelationships have to be maintained in using a French Bender, theprocess can take an extremely long time and its use requires a greatdegree of physician skill to accomplish an accurate final product. Evenstill it is difficult to achieve a well-shaped rod using the FrenchBender. Accordingly, various ways have been attempted to overcome thelimitations of the current technology.

A number of manual benders are described in the art. In U.S. Pat. No.5,113,685 issued May 19, 1992 to Asher et al, there is described anapparatus for use in bending rods and plates to the spinal columncomprising an elongated bar with a variety of bending angles for bendingmore angles than the French Bender. However, this device is hard to useand provides no means for determining the six degrees of spatialrelationship that each bend must make. In US patent application2006/0150699 published Jul. 13, 2006 to Garner, et al, there is aninstrument and method for bending rod using a lever pliers type devicehaving bearing surfaces. In addition, the angle of bend can bedetermined by use of a gauge that indicates angle bend by degree of gripmovement. While this device may be easier to use, it does not aid indetermination of the other degrees of freedom either in calculating themor in making the final bends.

An automatic method designed for pre-surgical formation of spinal rodsis disclosed in US patent application 2005/0262911 published Dec. 1,2005 to Dankowicz, et al. An automatic series of shaping steps is“imposed” on a rod from an input mechanism for producing the desiredmulti-dimensional bent shape. One problem with this device is that itrelies on a pre-surgical determination of the points at which bendsoccur to determine the final shape of the rod. While it is possible toanticipate where the anchors might ideally end up and occasionally becorrect, surgical implantation of attachment points is as much art asscience so a preformed rod may not be accurately produced when comparedto the anchor means as they are actually installed in the spine. Thiscan lead to a highly problematic circumstance in which the surgical sitehas been opened and the surgeon has a rod that does not fit theattachment points. Further disadvantages are that the device is largeand that some surgeons still would prefer a manual means of producing arod during surgery because of the ability to make minute adjustmentsbased on feedback during surgery.

Effort has been directed to computer-aided design or shaping of spinalrods, but these efforts have been largely unsuccessful due to the lackof bending devices as well as a lack of understanding of all the issuesinvolved in bending surgical devices. For example, an article entitled“A pilot study on computer-assisted optimal contouring of orthopedicfixation devices,” Computer Aided Surgery, 1999; 4 (6):305-13, indicatedthat overcoming these problems would be difficult if not impossible.

Image guided surgical systems, for example, devices produced byBrainLAB, as well as three dimensional digitizers are already in the artand some are already FDA approved for use during surgery. These devicesare fairly commonly used by some physicians in the operatingenvironment. By moving the digitizer through space or inputting aparticular point in space, a map can be produced of spatialrelationships. In U.S. Pat. No. 6,400,131, issued on Dec. 31, 2002 toLeitner et al., there is described a contour mapping system applicableas a spine analyzer and probe. The device is disclosed as being used todetermine the curvature of the spine while standing and contour mappingof the spine in the intact (non-surgical) patient.

Accordingly, a means for designing and forming a surgical linkingdevice, especially for linking bony parts of the body, for use in asurgical orthopedic procedure such as the attachment of a spinal rod,that is accurate, quick and takes the various input characteristics intoaccount for the specific implanted device as actually needed would be ofgreat value during an orthopedic implant surgery such as spinal surgery.

SUMMARY

In one embodiment there is a system for shaping a surgical linkingdevice for attachment to a selected bony body structure having at leasttwo linking device attachment elements comprising:

a) a means for determining the relative spatial location of at least oneof the attachment means and the bony structure;

b) a means for converting the relative spatial location into a digitalformat;

c) a computer capable of receiving this digital format and using therelative spatial location to determine one or more shape locations inthe surgical linking device, each shape location having one or more of ashape angle and shape rotation at each one or more shape locations suchthat shaping of the surgical linking device will enable the surgicallinking device to attach to the bony body structure using the attachmentelements; and

d) a means for delivering the determined shape information to a computeroutput.

In yet another embodiment there is a surgical linking device on aselected bony body structure comprising:

a) placing at least two linking device attachment elements on the bonybody structure at desired locations;

b) digitally determining the relative spatial location of at least oneof the bony structure and the attachment elements;

c) transferring the digitized information to a computer which determinesinformation of one or more of:

-   -   i) one or more of the location, angle and rotation of shapes in        a selected surgical linking device that could be made in order        for the linking device to be attached to the bony structure        using the attachment elements;    -   ii) one or more adjustments to the position of or addition to        the attachment elements that could be made so that a selected        preformed, partially preformed or a minimally shaped surgical        linking device can be attached to the bony structure with the        attachment elements;    -   iii) one or more mathematical adjustments to the digitally        rendered position of the attachment elements so that the final        shaped surgical linking device, once attached to the bony        structure, will correct or alter the shape of the bony        structure(s);

d) delivering the computer determined information to a computer output;

e) using the information from the computer output to perform one or moreof:

-   -   i) selecting a preformed or partially preformed surgical linking        device    -   ii) shaping a surgical linking device with a device that        measures one or more of the shape location, shape angle and        shape rotation; and    -   iii) adjusting the position of or adding to the attachment        elements; and

f) attaching the surgical linking device to the attachment elements.

Yet another embodiment includes a device for bending a surgical linkingdevice, in which the device is particularly suited for manual operation,comprising:

a) a lever for bending the linking device; and

b) at least two bend measuring means selected from the group comprising:bend position measuring means, bend angle measuring means and bendrotation measuring means.

Another embodiment of the invention includes a device for determiningthe rotation for placing a bend in a surgical linking device comprising:

a) a circular gauge indicating the degrees of rotation; and

b) a means for positioning the device on the surgical linking device oron a means for bending the linking device such that the gauge alignswith any bends in the linking device.

Yet still another embodiment is a means for determining the selection ofa preformed surgical linking device for use in attaching to a selectedbony body structure having at least two linking device attachmentelements comprising:

a) a means for determining the relative spatial location of eachattachment elements;

b) a means for converting the relative spatial location into a digitalformat

c) a plurality of preformed surgical linking devices;

d) a computer having selected spatial information about the preformedinking devices wherein the computer is capable of receiving the digitalformat in b) and using the digital format to determine if one of thepreformed surgical linking devices fits the attachment elements and ifthere is none that fit, if one or more attachment elements could beadjusted in relative location such that one of the preformed surgicallinking devices could be selected and fit the attachment elements; and

e) a means for delivering the determined attachment elements adjustmentsand selected preformed linking device to a computer output.

A further embodiment contemplates a method for placing multiple bendswith 6 degrees of freedom in a surgical linking device comprising:

a) establishing a starting point on the device;

b) holding the device relative to the starting point;

c) moving the device and measuring away from the starting point toestablish a second point on the device for placing a bend with 6 degreesof freedom; and

d) repeating steps b) and c) using either the starting point or thesecond point to hold from until the multiple bends are completed.

Another embodiment of the present invention is a process for producingone or more shapes in a surgical linking device comprising:

a) a digital process for determining the desired spatial parameters ofthe shapes to be produced; and

b) a shaping process linked to the digital process wherein the shapingprocess applies the spatial parameters to the surgical attachmentdevice, in which the shaping process is particularly suited for manualimplementation in the surgical operating room.

In yet another embodiment, a method is provided for shaping a surgicallinking device for engagement to a plurality of attachment elementsengaged within the selected bony body structure, each of the attachmentelements having an engagement portion for engagement with the shapedlinking device, in which the method comprises:

a) providing digitized data for the location of the plurality ofattachment elements;

b) determining a tolerance range corresponding to an acceptable distancethat the shaped linking device is from the engagement portion of eachattachment elements;

c) developing a curve function to approximate the location of each ofthe plurality of attachment elements;

d) calculating the location of the linking device shaped according tothe curve function at the location of each of the plurality ofattachment elements;

e) calculating an error based on the difference in the calculatedlocation of the linking device and the location of each of the pluralityof attachment elements;

f) determining if the error exceeds the tolerance range and if sodetermining a higher order curve function;

g) when the error falls within the tolerance range, generating a bendcurve having a discrete plurality of bend points using the curvefunction, the discrete plurality of bend points being distributed at apredetermined distance;

i) generating a revised bend curve with the remaining bend points; and

j) generating bending instructions to be performed on the linking deviceby a bending tool at each of the remaining bend points.

In another aspect of the invention, a digitizer probe is provided thatis configured to temporarily mate with the head of an implant. The probeincludes a shaft accessible beyond the implant that can be used to fixthe location of the implant when determining the bending protocol for arod, plate, or elongate member to engage the implant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a through 1 d depict a surgical rod and various bends with 6degrees of freedom.

FIG. 2 depicts three vertebrae each with a surgical rod attachmentscrew.

FIG. 3 depicts three vertebrae with a bent surgical rod attached to thethree rod attachment screws.

FIG. 4 depicts a front view of a rotation gauge for attaching to asurgical rod.

FIGS. 5 a and 5 b depict surgical rods with ruled markings.

FIG. 6 depicts a small hand device for bending a surgical rod and havinga means for measuring location, rotation and angle bend.

FIG. 7 is a perspective view of a dual lever surgical rod bendingdevice.

FIG. 8 is a side view of a dual lever surgical rod bending device.

FIG. 9 is a view of a dual lever surgical rod bending device with thelevers in the open position.

FIG. 10 is an end on perspective which allows view of the fulcrum means.

FIG. 11 is a flow diagram of an embodiment for determining bendinformation.

FIGS. 12 a-h show a comparison between the IdealScrewPositions in the XY(coronal) plane for an exemplary implant and the calculated positionsaccording to one example of the curve fitting approach of the presentinvention.

FIGS. 13 a-f show a comparison between the IdealScrewPositions in the XZ(sagittal) plane for an exemplary implant and the calculated positionsaccording to one example of the curve fitting approach of the presentinvention.

FIG. 14 shows a comparison between a calculated bend curve and a thecurve after “smoothing” according to one aspect of the presentinvention.

FIGS. 15 a-k shows a sequence of bend curves in the XY and XZ planeswith successive bend points eliminated to simplify the bend curve.

FIG. 16 is a representation of a graphical user interface (GUI) forpermitting user input and displaying information to the user during theoperation of the system of the present invention.

FIG. 17 shows the GUI of FIG. 16 after a bend curve has been calculatedfor a particular spinal construct.

FIGS. 18 a-d show a particular bend instruction as implemented using thebending tool shown in FIG. 7 .

FIG. 19 shows a rod bent according to the bending instructions displayedon the GUI shown in FIG. 16 .

FIG. 20 is a side view of a poly-axial implant with a digitizer probeaccording to one embodiment of the invention engaged hereto.

FIG. 21 is a top view of the interface of the digitizer probe with thehead of the implant shown in FIG. 20 .

FIG. 22 is a top view of an alternative interface of the digitizer probewith the head of the implant shown in FIG. 20 .

FIG. 23 is a side view of the interface of the digitizer probe with analternative implant.

FIG. 24 is a side view of a digitizer probe according to anotherembodiment engaged to an implant.

DETAILED DESCRIPTION

The present invention refers to a method for improving the shaping of asurgical linking device, for example, by bending. First, by digitallycalculating appropriate shapes such as bends in 6 degrees of freedom(three dimensional) and then outputting that information to the surgeonor other medical personnel or to a bending device, a linking device canbe easily and quickly shaped by casting, bending or the like. Second, adevice is disclosed for quickly and easily taking the input from adigitally calculated means, or other similar means, and manually shapinga precisely bent or shaped linking device. Accordingly, the time spentin surgery bending linking devices can be greatly reduced thus improvingthe chances of a successful operation without complications as well asreduce the cost of such an operation, for example, from rebending orbending a second device. Since a significant portion of time is spent inbending and in some cases rebending such devices, taking minutes to anhour or more off the time to bend a linking device correctly is animportant advance in the art.

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail specific embodiments, with the understanding that the presentdisclosure of such embodiments is to be considered as an example of theprinciples and not intended to limit the invention to the specificembodiments shown and described. In the description below, likereference numerals are used to describe the same, similar orcorresponding parts in the several views of the drawings.

It is understood that the term “coupled”, as used herein, is defined asconnected, although not necessarily directly, and not necessarilymechanically. The term “bending” refers to the act of forcing, or thelike, a linking device from a first position at a particular point to asecond angular or curved position at that point in three dimensionalspace. Six degrees of freedom are considered in bending a particulardevice once the location of the bend is determined. In general, once theposition of the placement of a bend is determined, then the angle of thebend and in many cases the rotation about a central axis may also bedetermined. In many cases a simple angular shaping is sufficient whilein others, such as is often the case for surgical rods, a rotation offaxis is necessary.

The bending is exemplified in the drawings which explanation follows. Asused herein “shaping” refers to not only bending but other methods oftaking the 6 degrees of freedom information generated with the presentinvention and producing a shaped device. In addition to bending, the useof extrusion, casting, deformation, molding and the like could beconsidered a means of shaping a particular device with the informationgenerated herein. See U.S. Pat. No. 6,749,614 issued Jun. 15, 2004 toTeitelbaum, et al., for an example of such material which could be usedto shape a linking device with the present invention methods.

A “surgical linking device” as used herein refers to those devices usedduring surgery to use to bind to a selected bony body structure to mend,stabilize, move, reshape, correct deformities or strengthen such asattachments made to bones. For example, surgical rods, surgical plates,surgical transverse connecting rods, surgical wire or surgical cable andthe like are used in surgery to mend, stabilize or correct breaks,correct deformities and the like in selected bones by attachment to twoor more attachment points. Such plates and rods usually are suppliedstraight in a number of lengths or preformed arcs and must be bent tofit their intended use. (See v2-Evren 2008 online catalog,www.v2evren.com.tr for examples of vertebral rods and connectors as wellas other orthopedic devices of a surgical nature). Typically, thesedevices are made of titanium or other extremely durable, stiff anddifficult to bend material. Rigid materials such as titanium,commercially pure titanium, stainless steel, cobalt chrome and the likecould be used. Other materials include flexible materials such as madeof PEEK or other appropriate plastics, graphite or the like, bumperedsystems and devices in both mono and multi diameter versions. Wherecasting or other shaping means are used, any rigid material suitable forsurgical use in these conditions can be used.

Additionally, useful are shape memory alloys, shape altering devices,materials with varying stiffness, biological materials and any syntheticmaterial with bioactive properties. In particular, the benefits of shapememory materials could be magnified by the processes described herein,especially when such processes are applied more than once on the samelinking device. The shape memory materials allow an initial shape basedon the location of the fixation points or facilitate rod implantationand final shape determination from the altered position. The linkingdevice can then be used to alter the orientation of the bonystructure(s) to help achieve the results of surgery. Other surgicallinking devices could include plates attached to specific body parts,both in the apendicular and axial skeleton, as well as cables and rigidclamps used to affix to and alter teeth and their alignment.

The French Bender is the surgical instrument of choice today to bendthese materials but it does so without regard to being able to measurethe 6 degrees of freedom of movement in any manner. Accordingly, theprocess of bending a surgical rod with a French Bender is laborious anddemanding, requires some degree of artistry and frequently requiresstarting over.

A “linking device attachment elements” refers to a means attached to abody structure designed to received the surgical linking device and holdit in place. Surgical clamps and screws are common examples of thesedevices. In the case of a surgical rod, a variety of surgical screws,bolts, and hooks are available to screw into the bone and or to hold therods in place. These include polyaxial screws, mono axial screws, fixedangle screws, iliac screws, sacral screws, lateral mass screws, bolts,laminar hooks and pedicle hooks. In additions, items such as staples, orplates that serve to hold one body part, can serve as an anchor to whicha linking device can be affixed onto the spine especially with anteriorplating systems. All these systems can be used together and furtherconnect up to similar anchoring plates.

Connectors such as axial, lateral and transverse connectors are usedwhile locking screws are often used to hold the linking device in place.Even further, the attachment elements could be devices added to themeans to change the attachment position. A screw attachment or “offset”,for example, could be used. In the practice of this Invention, thedevices and methods of this invention anticipate use when there are atleast two and frequently three or more attachment elements correspondingto each surgical finking device. Multiple differing types of attachmentelements could be used in a single installation. In addition, in thecase of plates, the attachment elements may be installed after theshaping of the plate based on the shape of the plate rather than theother way around.

“Determining the relative location of each attachment elements and bonystructure” refers to understanding the spatial relationship between thebony structure and any points of attachment so that a linking devicesuch as a surgical rod can connect between the attachment points giventhe proper shape of the device. The relative location can be obtainedwith currently available image guidance devices such as threedimensional digitizers (such as the Polhemus Patriot) which can be usedsimply by engaging the device at several attachment points or along thebony structure and letting a computer in the device or elsewheredigitize the information. A partially manual method could be done, forexample, by photographic means such as x-ray or regular photography andthe spatial relationship determined away from the patient. Such a methodmight need a plurality of photographs but given this explanation is wellwithin the skill in the art.

From the determination of the relative spatial location, the informationcan easily be digitized either automatically, as is the case with thethree dimensional digitizer, or by entering hand calculated informationinto a computer or the like which then stores the information digitally.Either way, the information is converted into a digital format which acomputer is capable of manipulating. Other devices could be optical, EM,image guidance systems, Shape Tape™, ultrasound, cat scans, and otherradiographic devices. The key is that information needs to be gatheredabout spatial relationships and that information is capable of beingobtained in a variety of ways. It is clear that the enumerated means orany other means which achieves the determination of the spatialrelationship can be used by one skilled in the art. In some embodimentsthe expression “determining the relative location of each attachmentmeans and bony structure” may also refer to making multipledeterminations after adjustments to the installation or attachmentsmeans are made. One skilled in the art will know when and how to makesuch multiple determinations.

Since structures such as the shape of the patient's anatomy, bonestructure, other devices in the area and the like may also need to beconsidered when determining the bend profile, the invention furthercontemplates that other structural information may also be created in adigital format for transfer or use by a computer. In one embodiment, thecontour or structure can serve as the input by itself, such as with anyplating system, where the input is the topography of the surface of thebody part, with this input being used to guide shaping the implant. Theattachment points are then driven through the plate after the plate isshaped, not prior, in as much as the information could be determinedsolely from surface anatomy and not the attachment points.

A computer such as a laptop, hand held device, desktop or other computerdevice can receive the relative location of the attachment elementsand/or the bony structure in a digital format. The computer thenprogrammed with the spatial information can determine the best way toshape, bend or the like, the linking device in order to fit theattachment elements. This determination of bends also takes intoconsideration the fact that other structures or the shape of thestructure being attached to may be in the way. For example, in a spinalprocedure, the shape of the vertebrae bones must also be considered.

The computer can be programmed to accommodate any number of parametersin determining the output or the final shape of the linking device. Inthis way, the goals of surgery can be assisted through the alteration ofthe shape of the linking device. Whereas in one embodiment, the shapedictated by the information above and not altered further could be usedto create the linking device, further alterations in the device's shapecan help to address, straighten, or alter abnormalities in alignment ofthe body part(s), create lessen or eliminate deformities, reduce orimpose changes in alignment or the addition or elimination of stresses.It is possible to couple the changes in different planes or simply applycorrection in one plane, rather than in another orthogonal plane. Thesemodifications of the shaping information that is outputted can beobtained through various means—visual, anatomic, guided by radiographs(intraoperative, preoperative, positioning films, etc.), guided by thematerial properties of the linking device and the plasticity and/orrelative location of the body part(s) being altered.

The computer need not have direct interaction with the device used forbending, in one embodiment. In other embodiments, it could input theinformation directly to the shaping device such as to a screen or othermeans such as to set the dials prior to shaping. The computer definesmathematically from the spatial location of the attachment means and thebony structure of the body, the heads of screws, surface of that bonybody part and the like, a curve which approaches these points in threedimensional space within the requirements and capabilities of theselected surgical linking device. The determined information can be usedto select a specific device, to place bends in an unbent or pre-bentdevice (or shape as needed) or to adjust the attachment means asdesired. In addition, a number of different shape solutions could beaccommodated such that the surgeon can use personal judgment inselecting the best shape solution.

The computer could further customize the output of the bend information.It could minimize the number of bends if desired (for example, with aquicker zigzag type design with greater bend angles at fewer bend pointsbut with potentially greater stress risers). In other embodiments itcould increase the number of bend locations to create a smoother design,since the more bend points the smoother the bend. One could limit theprogram or the device to specific angles so that all angles would beabove, at or below a particular value. It could also limit the choicesto incremental choices such as every 5 degrees of bend or rotation ordistances to a few millimeters. A simple design connecting points couldbe achieved as could a more complex design as desired. The computercould determine the size of the device, can determine if the attachmentsmeans can be adjusted or added to with offsetting devices (and thereforeincrease or decrease the number of bends to attach the points). In oneembodiment, the program can be used to see if the attachment points canbe used with a pre-bent device either without modification or withadjustment of the attachment elements or the addition of spatialoffsetting devices. The computer could also pick shapes that simplifythe shape of the linking device or improve its biomechanics.

A first step in bending a linking device is to determine a bendlocation. The bend location is a point on the linking device where thebend will occur. It can be measured from a starting point, for example,1.5 cm from the distal end of a surgical rod, or it can be determined byselecting from a set of fixed points on the device. For example, ruledmarkings every centimeter on a rod or other device could be marked aspoint 1, 2; or as 1 cm, 2 cm, etc., and the output of the computerdeliver the fixed point. In another embodiment, the device is held inplace and moved a given distance from the point held as a referencestarting point.

The bend angle is the degrees that the device is bent away from aparticular axis or plane. The bend can be accomplished as a single bendor it can be a multiplicity of bends as described above. In general, thebends will be from just greater than zero to 180 degrees off ofstraight. In many embodiments the bend angle is 90 degrees or less. Ingeneral, the maximum bend angle will be determined by a number offactors including the particular use, the surgeon's typical practice,the materials employed and the like. In addition, the angle of rotationoff of the direction the device was going could be determined. So, forexample, a surgical rod could be angled from zero to 360 degrees off ofthe zero axis of the original direction of the rod in addition to thebend. Thus a bend of, for example, 45 degrees with a rotation of 15degrees, 2 centimeters from a starting point could define a particularbend output. The distance rotation and bend angle after determination isthen delivered to a computer output. The output can be a paper output, aGUI (Graphic User Interface) or the like, such that a user can read theinformation and begin the process of bending a device. In oneembodiment, the information is delivered directly to the bending device.

The means for placing a bend in the surgical linking device can, in oneembodiment, be accomplished by one or more manual devices. Handmeasuring distance, a rotation disk (as shown in FIG. 4 ), and then abending device for bending to an angle could allow the bending withthree interactive devices. Likewise, the device shown in FIG. 7 could beused to set all three parameters on one device. For a device that onlyneeds 4 degrees of freedom, the computer needs only produce distance andbend angle and the various devices above either singly or one singledevice could be used. Rotation in this case could be set at zero.Further, such as in the case wherein the output of the system determinesthat a pre-bent rod could be used, the output of all of the parametersexcept distance could be zero. The system could simply determine whichlinking device that should be chosen, with or without the need tofurther manipulate the screw locations or add additional offsettingdevices. In this case, no bends may need to be made.

Surgically, the method for installing the surgical linking device on abody bony structure using the present invention, in one embodiment,could be started by placing at least two linking device attachmentelements on the body structure at desired locations. Then the spatialrelationship of the attachment elements could be determined in a digitalmanner. The digitized information would be transferred to (includingcalculated by) a computer which determines one or more of the following:one or more of the bend location, bend angle and bend rotation such thatupon making the bends the device will fit the installed attachmentmeans; it could also determine that one or more adjustments or additionsto the position of the attachment elements could be made so that onecould select a preformed or partially preformed device or that a devicecould be bent with fewer bends or no bends at all to fit the attachmentelements. The computer calculates and delivers the information to acomputer output. The output could be used to perform one or morefunctions during surgery, namely selecting a preformed or partiallypreformed surgical linking device; placing one or more bends asdescribed above in the device or adjusting the position of theattachment elements or placing an addition to the attachment elements.After the proper selection and bending the surgical linking device isattached to the attachment elements.

The advantages and uses of the computerized means for determining theshape of a surgical linking device are several. It allows for thefacilitated implantation of preformed whole rods or segments, and theability to define the size and shape of the component pieces of amulti-component linking device. The linking device can aid a surgeon inthe formation of the desired end result rather than the situation asconfronted. The linking device can be designed and formed based on theintersection of this desired end result, the current position of theanatomy, and the location of the affixing points. This can be used tocontrol the reduction of fractures and deformities by defining theamount to translation, rotation and or angular correction and alteringthe shape of a linking device to achieve the result. Further, it can beused to correct spondylolisthesis.

In another embodiment, this method could be used to define the resultantrod and thus help form, obtain and/or hold the correction required inperforming an osteotomy or other type of corrective technique used insurgery. The linkage device can be implanted without any static loadimparted to the body, or with a predefined load which can aid inadjusting deformities or set the location of a flexible system. Onecould determine how the anatomy moves or has moved or changed, and onecan determine the amount of implant manipulation needed to gain theanatomical change desired. (For example, using x-rays in the OR andcomparing them to images taken prior to surgery it is possible to figureout how much to alter the shape of the linkage device in order toachieve the straightness the patient can physically achieve by bending).In one embodiment, one linkage device could be made which would resultin completely obtaining the desired end result. In another embodiment,successive intervening steps could be made (i.e.—multiple linkagedevices incorporating successively greater amounts of deformitycorrection) to allow a slower, more gradual correction of the deformity.As all people's anatomy changes to some degree when lying in an OR tableversus the upright position, the present invention could be used toaccount for this change.

Although in one embodiment, the rod can be formed quickly at the time ofsurgery, this is not required. One could immediately implant or deferthe linking device implantation such as to let ongrowth or ingrowthoccur then implant the formed rod in a delayed fashion. Further, thissystem is ideal to custom design large percutaneous implants. As well,it could be used to design a transverse connector that joins two or morelinking devices or any other type of implant that could benefit fromlinking. Further, it can be used to accommodate an easy way to extendthe linking device should this be required in the future, as the endconfiguration and angle of one embodiment of this device is know at thetime of production and therefore this additional step (which is usefultypically in a delayed fashion months to years later) could further beincorporated.

Bending is preferably accomplished manually at the surgical site byknown means but in the alternative can be accomplished with noveldevices of the present invention. Novel bending devices all comprise atleast one lever, namely in the form of a bar or long arm that can beused to bend an object around a particular pivot point. With one leverthe object to be bent is forced with the aid of the lever. In otherembodiments, there is a pair of levers that can bend around a fulcrumthat is a point or device that will aid in bending the device around.

Devices such as the French Bender have no means for determining any ofthe bend parameters discussed above when bending a surgical linkingdevice. The present bending device includes means for determining atleast two of those parameters. In one embodiment, the two parameters arelocation and bend angle. In another embodiment, the device measureslocation, bend angle and bend rotation. Each lever can have a handledisposed at a distal end to aid in grabbing the lever and leveraging itduring use.

The means to measure the spatial parameters can measure a continuouslocation or angle or in other embodiments the measurement means canmeasure incrementally (i.e., non-continuously). So, for example, thelocation can measure in half centimeter, one centimeter or otherincrements, while the angle of bend or rotation could be measured infive degree increments or the like. Continuous measurement or click stopmeasurement could be used with each measuring means individually ormixed as desired. Greater accuracy may be obtained by continuous ratherthan incremental movement, but the choice would be up to the user andtype of bender device employed.

In addition, the device may be capable of fixedly holding the linkingdevice. In this manner the bending device can use another means toadvance the linking device to the next bending location based on thecontinuous or click stopped measuring means. By fixedly holding thelinking device, the measurements can be made accurately from a specificstarting point adding a new starting point after each bend or using theoriginal starting point. For example, a bend could be put at onecentimeter and three centimeters from a starting point. In anotherembodiment, a bend is at the starting point and the next bend a fixeddistance from the starting point. In another embodiment, by holding thelinking device the linking device could be advanced based on ruledmarkings on the linking device instead of ruled markings on the bendingdevice. Where on the bending device, there could be regular stoppositions that are fixed or in the alternative, continuous adjustment ofdistance.

In general, one of the embodiments of the present invention is theprocess for producing bends in a surgical linking device which iscomprised of two separate processes linked to each other. The firstprocess is the digital process for determining the spatial parameters ofone or more bends. The second process is the manual process of shaping asurgical linking device that applies the location, angle and rotationparameters determined in the first process. The complete linking ofthese two processes is facilitated by the introduction of the noveldevice of the present invention. The link can be the surgeon or otherindividual who takes the computer output and applies the result to thelinking device, whether manually or by an automated bender or contouringdevice. For example, in another embodiment, the process or method forthe determination of the linkage device, including selection, alterationof fixation points or location, etc., could be applied to any of a hostof novel devices which would be necessary to help in the formation ofthe actual device. This would be ideal as materials used in orthopedicsurgery change over time, such as described in U.S. Pat. No. 6,749,614issued Jun. 15, 2004 to Teitelbaum, et al.

Now referring to the figures, FIGS. 1 a through 1 d depict various bendsin a surgical rod linking device. FIG. 1 a depicts a rod with a firstbend 11 and second bend 12. This depiction has the rod 10 lying in oneplane and the distance between bend 11 and bend 12 is shown as D. Byindicating a distance D from bend 11, one can obtain the location of thesecond bend 12. The starting point for measurement could be either frompoint 13, the first rod end or bend 11. The starting point for bendlocation can stay with the original point for subsequent bend locationdeterminations or can move with each bend location determination. So forexample, bend 12 could be the starting point for the next bend locationdetermination. In FIG. 1B, a single bend 15 is shown with an angle A.The angle A is the second determined parameter of the present invention.FIGS. 1 c and 1 d depict a bent rod with at least one bend that has beenrotated R degrees from the initial plane of the rod. Second end 19 isalso depicted and in FIG. 1 d the view is head on to the middle sectionof the bent rod 10. While a surgical rod 10 is depicted for clarity, asurgical plate or other surgical linking device could also be orientedand bent or shaped in a similar manner.

FIG. 2 depicts body structure vertebrae 20 laid out in perspective view.Each vertebrae 20 has had attachment elements, vertebral screw 21installed for the purpose of installing a surgical rod. Note that whilenormally rods are installed in pairs one set of screws 21 is shown forsimplicity's sake.

FIG. 3 depicts a bent surgical rod 30 which has been attached to theattachment elements 21. Also depicted is bend angle A and rotation angleR at which the rod has been bent to accommodate the positions of theattachment screws 21.

FIG. 4 is a rotation gauge 40 which may be fitted on the end of oraround a surgical linking device, for example, the rod 10 depicted inFIG. 1 . The rod 10 fits into hole 41 and then if the rod is rotated tothe degree markings 42, a rotational bend of a selected angle can beachieved. This device could be fixedly attached to a bending device asfurther taught herein.

In FIGS. 5 a and 5 b surgical linking rods 50 are shown. These rods arenormally cylindrical but first end 51 is squared off to accommodate atool or grabbing means or the like. Any number of other endconfigurations could, in addition, be used that can be firmly held orgripped. The gauge 40 from FIG. 4 could also be attached to this end.These surgical linking rods 50 also show either distance markings 55 toindicate the distance for a bend location. In the case of FIG. 5 brotational markings 56 are available not only for distance measurementsbut for rotational measurements as well.

FIG. 6 depicts a simple hand bending device 60. By squeezing handles 61and 62, rod 10 can be bent around a fulcrum (not seen). The rod is notheld in place but the rod 10 is moved and by matching distance markings63 on device 60 with rod distance markings 55 a clear location on therod 10 can be determined. Rotational gauge 40 is installed and bymanually rotating the rod 10 one can obtain a desired rotation. Whilethe rotation is marked in intervals, this embodiment allows freerotation of rod 10 thus infinite rotational angle. The bend angle ismeasured by angle gauge 65. Angle gauge 65 measures the angle based onhow close the handles 61 and 62 approach each other during the operationof bending rod 10.

FIG. 7 is a perspective view of a more detailed bending device 70 withless manual manipulation of the rod 10. A first lever 71 is shown as islever handle 73 designed for grabbing the lever 71 manually. Likewise,lever 72 is shown with handle grip 74. Grip 74 has rod pass through 78so that an infinitely long rod 10 can be used with this particularhandle as well as steady the rod during the bending process with bender70. The user of the device grabs both handles and opens the handles tobend the particular surgical rod 10 by picking an angle on the anglegauge and closing the handles 71 and 72 together. The device in otherembodiments could be produced to bend the rod during the handle openingmovement as well. The rod 10 moves through mandrel 80 and in betweenmoving die 81 and fixed die 82. A better view of the dies is in FIG. 10. The surgical rod is bent between the two dies 81 and 82. Gauges on thedevice allow the user to manipulate the surgical 10 rod in order todetermine bend position, bend angle and bend rotation. The surgical rod10 is held in place by collet 75. By sliding slide block 76, alonghandle 72, the surgical rod 10 can be moved proximally and distally inthe bending device 70. Position is measured by click stops 77 at regularintervals along handle 72. Each click stop 77 is a measured distancealong the handle 72 and thus moving a specific number of click stops 77gives one a precise location for the location of a surgical rod 10 bend.

The bend angle is measured by using angle gauge 85. Gauge 85 has ratchetteeth 86 spaced at regular intervals. Each ratchet stop represents fivedegrees of bend angle. Thus the user can bend a surgical rod 10 in fivedegree increments with the particular bend angle gauge 85 as the handles71 and 72 are opened and closed. The bend rotation is controlled by adial in the form of collet knob 90. By rotating collet knob 90 eitherclockwise or counterclockwise the user can set a particular rotationangle. The collet knob 90 is marked with regular interval notches 91 butthis particular embodiment is continuously turn able and thus hasinfinite settings. Once a user turns knob 90 the user can set the knob90 at a particular marking 91 or in between or the like to determine aparticular angle rotation to a high degree of accuracy.

In this particular embodiment, once the rod 10 is locked in place withcollet 75 if there is enough room on the lever 72 to move the slider 76distally or proximally then the rod 10 can remain fixedly attached tocol let 75. Should a longer area need to be bent, then the rod 10 can beunlocked moved and relocked and measurements start from the newposition. Merely adding the positions together using the informationsupplied by the computer output would be an easy task with the presentinvention.

FIG. 8 depicts the bending device 70 in a side view. In this view onecan clearly see the rod 10 has bend 92. FIG. 9 shows a side view whereinhandle 71 is open in preparation of making a second bend in rod 10. Bendgauge window 96 shows bend angle pin 97 which has engaged 2 teeth 86 inpreparation for placing the second bend. As can be seen in this view therod 10 has moved distally since slider 76 is in a more distal positionthan shown in FIGS. 7 and 8 . First bend 92 has moved distally as welland upon closing of levers 71 and 72 a second bend will be placed in rod10.

FIG. 10 shows a head on view of the device 70. In this view, the rod 10can clearly be seen in bent position between moving die 81 and fixed die82. The moving die 81 allows for free movement of rod 10 and the fixeddie 82 allows for relatively easy bending of rod 10.

FIG. 11 depicts a flow chart of a particular embodiment of the operationof the computer means in combination with the device of the presentinvention. The first step 110 in the process is the installation of alinking device attachment elements to a body structure. In otherembodiments, for example for use with a surgical plate, the first stepis to determine the surface spatial relationship of the bony structureand then using that spatial information to determine the shape of thesurgical plate. Once the plate is placed on the bony structureattachment means are positioned through the plate and into the bonystructure. The linking device such as screws for use with surgical rods,which to some degree adjustable then determines where the linking devicewill be positioned.

The next step 111 is the determination of the spatial relationship ofthe attachment elements into a digital format. This is done not onlytaking into account the position of attachment, but also taking intoconsideration any body structures which may intervene in the process. Itwould not be useful if a part of the vertebrae were in the way of aparticular bend solution because the resulting bent rod would not fitthe attachment points because of body structure interference. Oneskilled in the art could easily make the appropriate adjustments to thecomputer calculation based on the disclosure herein.

Next, the computer with the possession of the digital format determinesthe bend parameters and or the device attachment elements adjustments instep 112. This step may also include the selection of a particularlinking device, the size it needs to be, or to select from a list ofpre-bent linking devices. Once a linking device is selected from thecomputer output parameters, the linking device is then, if necessary,bent or shaped and or the attachment elements adjusted in step 113.After the appropriate bends have been made, the linking device isattached to the attachment elements in step 114.

Step 112 of the flowchart of FIG. 11 entails first determining amathematic representation of a linking device (such as a rod or a plate)that will fit each of the attachment means in situ. Thus, according toone embodiment, a software program implements a curve fitting algorithmthat is adapted to approximate a smooth curve spanning between theattachment points, with the curve falling within an acceptable error atthe location of each attachment point. The software program starts withthe digitized data establishing the three-dimensional position of eachattachment point. In order to simplify the curve fitting protocol, thepresent invention contemplates that the three-dimensional data are usedto establish the attachment points in two orthogonal planes—the sagittalor XZ plane, and the coronal or XY plane. As is known in this field, thesagittal plane corresponds to a vertical plane passing through the spinefrom the front to the back of the patient. The coronal plane isperpendicular to the sagittal plane and extends side to side through thepatient. The division of the 3D coordinate system into two 2D planes maybe used in one embodiment as described in detail herein, but would notbe required for the functioning of the system. In another embodiment, asingle 3D curve fitting program is employed.

It is thus an object of the software program to derive a curve in eachplane that fits the actual position of the attachment points in situ. Inmost cases, the curves in the sagittal and coronal planes are complex,meaning that the curves will typically incorporate multiple inflectionpoints. Thus, it should be understood that a straight line or even anarcuate line will usually be inadequate to fit the true position of theattachment points, especially as the number of attachment pointsincrease. It can be appreciated then that a first or second orderpolynomial expression for a curve in either plane will rarely besufficient to model the three-dimensional representation. It can befurther appreciated that an exact curve fit is unlikely, even if thepolynomial is extended to a very high order.

The present invention accounts for these difficulties by incorporatingan acceptable error between the actual three-dimensional location of anattachment point and its mathematical representation. This error isacceptable if kept within certain constraints because of the ability ofthe surgeon to manipulate the linking device, inherent characteristicsof the attachment points and linkage device and even the spine whencompleting the spinal construct. For instance, where the attachmentpoint is a bi-axial or multi-axial bone screw, the head of the screw canbe toggled or pivoted so that the rod-receiving channel of the screw canbe oriented to receive a linking device, such as a spinal rod. Inaddition, in many attachment devices, the interface point is the channelof a yoke or saddle-shaped structure, which provides variability to theangle between the rod and the head of the bone screw. In some devicesthe screw permits variation in vertical orientation relative to thebone. Each of these factors contributes to an acceptable error ortolerance. Other factors that may be considered in deriving theacceptable error include the material of the bone screw and the likingdevice, the cross-sectional shape of the linking member, the distancebetween attachment points, and the size of the bone screw and thelinking member.

In accordance with the dual plane approach of the present embodiment,the error, or more appropriately tolerance, is established in eachplane. For illustration purposes these tolerances can be designatedToleranceXY and Tolerance XZ. The ToleranceXY in one embodiment can begiven by the expression:ToleranceXY=tan(0.5*MaxHeadAngle)*(distance from the end of the fixedshaft of the screw to the ideal rod location).

The term “MaxHeadAngle” corresponds to the maximum angle through whichthe head of a bone screw can toggle or pivot. For a typical polyaxialscrew, that angle can be in the range of 50°. The last term in theexpression essentially corresponds to one side of a triangle indicatingthe depth of the saddle of the screw that can accommodate the linkagedevice.

The ToleranceXZ corresponds to the inherent amount of verticaltranslation that the screw head can accommodate. For example, a screwhead having a deep yoke channel, the vertical translation may be as muchas several millimeters and in some systems, especially in those in whichthe heads resemble posts, the number can be much more.

The two tolerance values, ToleranceXY and ToleranceXZ, are determinedfor each attachment point—i.e., for each bone screw. The tolerancevalues provide the measure for determining whether a particularpolynomial representation of the rod shape is sufficient. Again, thesetolerance values may take into account all or some of the factorsdiscussed above.

In accordance with one aspect, the curve approximation for each planestarts with a first degree polynomial, the lowest order possible, andproceeds to increase in order until a curve is developed that satisfiesall of the tolerance values. Thus, an initial approximation for a curvefit starts with the equation:y=P ₁ *x ¹ +P ₂ where x and y define the location of the curve in thecorona or lateral plane with y representing the left to right locationof the curve function and x corresponding to the head to foot locationalong the length of the spine, and P _(i) are coefficients.

A similar relationship is applied to find the value of z, namely thelocation of the curve in the sagittal, or front-to-back-plane. Further,in other embodiments of the present invention, other curve fittingalgorithms are used to establish an equation for the linking device, allincreasing in their complexity as they minimize the error between thedetermined curve and the attachment points.

Invariably, this first order polynomial will be insufficient to providea curve linking all of the attachment points. The order of thepolynomial is then successively increased according to the followingequation:y=P ₁ *x ^(N) + . . . +P _(N) x+P _(N+1).

For each N-th order polynomial, the error in the two planes (XY and XZ)is calculated using a least squares approach. To determine this error,an IdealScrewPosition value is obtained for each attachment point,namely (X_(screw), Y_(screw), Z_(screw)), which corresponds to the ideallocation of the spinal rod within the head of the screw. In oneembodiment, the Ideal Screw Position values may correspond to thedigitized data for each screw in situ, in a manner as described above.In accordance with one aspect of the invention, the two errors ErrorXYand ErrorXZ are defined by the distanced from the calculated curvefunction to the IdealScrewPosition at two points on either side of theIdeal Screw Position according to the following relationships:

${ErrorXY} = \frac{\begin{matrix}\left( {{\left( {y_{{curve}1} - y_{{curve}2}} \right) \star x_{screw}} + {\left( {x_{{curve}2} - x_{{curve}1}} \right) \star y_{screw}} +} \right. \\\left. \left( {{y_{{curve}2} \star x_{{curve}1}} - {x_{{curve}2} \star y_{{curve}1}}} \right) \right)\end{matrix}}{\left( {\left( {x_{{curve}2} - x_{{curve}1}} \right)^{2} + \left( {y_{{curve}2} - y_{{curve}1}} \right)^{2}} \right)^{1/2}}$${ErrorXZ} = \frac{\begin{matrix}\left( {{\left( {z_{{curve}1} - z_{{curve}2}} \right) \star x_{screw}} + {\left( {x_{{curve}2} - x_{{curve}1}} \right) \star z_{screw}} +} \right. \\\left. \left( {{z_{{curve}2} \star x_{{curve}1}} - {x_{{curve}2} \star z_{{curve}1}}} \right) \right)\end{matrix}}{\left( {\left( {x_{{curve}2} - x_{{curve}1}} \right)^{2} + \left( {z_{{curve}2} - z_{{curve}1}} \right)^{2}} \right)^{1/2}}$

If any of these error values exceeds the tolerance values(ImplantToleranceXY or ImplantToleranceXZ), then the order of thepolynomial is increased and the least mean squared function and errorcalculations are re-run. In some embodiments, the tolerance values areaugmented by some small dimension, for instance 1 mm, to help simplifythe curve function and therefore eliminate bend points when the finalbend curve is created. In other words, it is typically desirable toreduce the number of bends that are needed to fit the implant locations,especially when performed manually. When applied otherwise, for examplein embodiments using non-manual bending devices or alternative materialsthat benefit from, reducing the number of bends may not be required orconsidered as advantageous to achieve a very smooth result. Theaugmented tolerance values can eliminate some bends that might otherwisearise with a tightly toleranced curve calculation.

In another aspect of the inventive procedure, steps may be taken toensure that the rod interaction with the head of the screw falls withina predetermined angle. This predetermined angle is based on the valueMaxHeadAngle, which is described above as the maximum permissible anglethrough which the head of the fastener/screw may toggle. It can bedetermined that the angle at which the rod crosses the head of the screwis defined by:Rod2ScrewAngle=arcsin(V _(screw) *V _(rod)), where the operator “*”signifies the dot product of the two vectors corresponding to theorientation of the screw and the approach angle of the rod.

In circumstances in which the rod crosses the head of the screw tooacutely (i.e., outside the boundaries set by MaxHeadAngle), thegenerated curve is altered in the immediate region of the screw using asegmental rod morphology which crosses within the correct range.Specifically, the curve function is altered by an amount R so that theRod2ScrewAngle function is equal to half the value of MaxHeadAngle. Inother words,V _(required rod) =V _(rod)+(R×V _(screw)), and(½×MaxHeadAngle)=arcsin(V _(screw) ·V _(required rod)).

The curve is then altered across the small segment adjacent theparticular screw as follows:D=V _(required rod)×(x ₂ −x ₁);y _(1new) =y ₁ +Dy _(2new) =y ₂ −D.

It is also contemplated that in using the least mean square approachdescribed above to determine the rod curve, some regions of theresultant rod may conflict with the surrounding anatomy. In extremesituations, this competition can be eliminated in part by simplydefining any anatomical points of concern at the same time that screwpositions are determined. In other words, the anatomy that must beavoided can be defined ab initio along with the locations of the severalbone fasteners/screws.

However, in the typical case, no strange anatomy is encountered. In thisinstance, it is common for the bone fastener head to project from thebony anatomy in to which it is anchored by a certain distance, usuallyabout 1 cm. When the bend curve is defined, all that is required is thatthe resulting rod position fall within a “safe region” which can bepre-defined as a few millimeters above and below a straight lineconnecting successive IdealScrewPositions. When the curve function wouldresult in the calculated rod position falling outside this “saferegion”, the curve is altered towards the straight line. In one specificembodiment, if any point along the calculated curve that extends above aline defined by the slope M of a line connecting screws at positions(X_(screw1), Y_(screw1), Z_(screw1)) and (X_(screw2), Y_(screw2),Z_(screw2)) exceeds a specified amount, the whole section of the curve(X_(curve1), Y_(curve1), Z_(curve1)) to (X_(curveN), Y_(curveN),Z_(curveN)) between the two screw positions is brought closer thestraight line. Thus,M=(zscrew2−zscrew1)/(xscrew2−xscrew1); andZcurve(1 to N)=½×(zcurve(1 to N)+zcurve1+[0M 2M . . . (N−1)M]).

In yet another embodiment, additional smoothing functions may be appliedto further smooth the overall shape of the resultant linkage device.

By way of example, FIGS. 12 a-h show the sequence of curve fittingpolynomials according to one embodiment of the invention, compared tothe ideal screw positions of a desired implant construct. The curvefitting for the coronal or XY plane follows the equation:y=P ₁ *x ^(N) ±P ₂ x ^(N−1) . . . P _(N) *x+P _(N+1).According to the specific embodiment, the coefficients for eachsuccessive order of the polynomial are:

1^(st) Order 0.0692 1.5576 2^(nd) Order −0.0315 0.5285 0.9663 3^(rd)Order 0.0003 −0.0374 0.5573 0.9579 4^(th) Order 0.0007 −0.0199 0.13810.1302 0.9334 5^(th) Order 0.0000 0.0021 −0.0374 0.2179 0.0385 0.89896^(th) Order 0.0000 −0.0008 0.0133 −0.1119 0.4007 0.0014 0.8104 7^(th)Order 0.0000 0.0001 −0.0024 0.0282 −0.1744 0.4974 0.0174 0.7641 8^(th)Order 0.0000 0.0001 −0.0018 0.0214 −0.1312 0.3593 −0.1697 −0.1664 1.0883

The polynomial expression for the curve fitting in the sagittal or XZplane is the same as that given above the XY plane, with thesubstitution of the variable z in lieu of the variable y. For thespecific example, the comparison of the calculated curve to the idealscrew position is shown in FIGS. 13 a-f and incorporates the followingcoefficients for the XZ curve polynomials:

1^(st) Order −0.0979 0.3041 2^(nd) Order 0.0395 −0.6738 1.0455 3^(rd)Order −0.0010 0.0603 −0.7752 1.0749 4^(th) Order −0.0011 0.0295 −0.2050−0.1293 1.1120 5^(th) Order 0.0001 −0.0049 0.0769 −0.4205 0.1183 1.20526^(th) Order 0.0000 −0.0012 0.0148 −0.0548 −0.0972 0.0526 1.0486

It can be noted that the curve approximation for the coronal XY planerequired an 8th order polynomial, while the approximation for thesagittal XZ plane only required a 6th order polynomial. It should beunderstood that the order of the polynomial necessary to closelyapproximate the ideal screw positions in the two planes will frequentlybe different.

For the XZ curve, FIG. 14 illustrates the effect of curve smoothingdescribed above. In particular, in the region of the curve between thescrew location 9.5 and last screw location 15.0, the calculated curveprovides a suitable contour to fit the four screws in that region.However, the curve segment between the screw locations 9.5 and 13.0 aremore exaggerated than necessary—i.e., the calculated curve falls welloutside the “safe region” around a straight line through the fourlocations. Similarly, the curve segment between the penultimate and lastscrew locations is slightly more exaggerated than necessary.

Thus, using the curve smoothing approach described above, the curvebetween the first two screw locations is flattened significantly, whilethe curve between the last two screw locations is flattened slightly. Inboth cases, the resulting smoothed curve more closely follows the slopeM of a line segment passing through the four screw locations.

With the smoothed curve approximations for the XY and XZ planes, thenext step is to determine where and how to bend a straight rod toachieve the desired shape. According to one aspect of the presentinvention, a computer-based system is provided that generates a sequenceof bend instructions. In the preferred embodiment, these instructionsare adapted to the particular bending tool, such as the tool 70described above in connection with FIGS. 7-9 . Thus, in the illustratedembodiment, the system of the present invention produces a list of bendsidentified by axial location along the rod, the amount of rotation aboutthe axis of the rod, and the magnitude of the bend.

In order to accommodate the bending tool, the system of the presentinvention seeks to break down the curve function generated above intomanageable line segments that can be readily handled by the bender. Ofcourse, each bending tool has its own inherent tolerances regarding thenature of the bends that it is capable of making. For instance, somebending tools can only make bends in a rod that are separated by 1-2 cm.In accordance with the present illustrate embodiment, the bending toolcan accept bends in ½ cm increments. Thus, the software of the presentsystem can determine the necessary bend angles at these ½ cm increments.In accordance with one embodiment, the software does generate bend datafor the minimum permissible increment, in this case ½ cm. However, it isexpected that the making a bend every ½ cm is too cumbersome and timeconsuming, and generally not necessary to produce a well-contoured rodfor implantation. In many cases, the surgeon will prefer a “simple”bend—i.e., one with the fewest number of bend points—versus a “smooth”bend—i.e., one that produces a smoothly contoured rod and thatnecessarily requires more bend points. In one feature of the invention,a GUI allows the surgeon to determine the bend type—simple or smooth—andin some embodiments to select a sliding scale between simplest andsmoothest bend type.

In determining the “simplest” bend, the object is to eliminate as manybend points as possible without compromising the overall shape of therod and the ability of the contoured rod to mate with the implanted bonescrews. In a first step, the bend point with the smallest bend angle iseliminated. In alternative embodiments, other bend points are chosenfirst, either arbitrarily, to spread the bend points apart or to limitthe number and size of bends at or near an attachment point and thelike. Regardless, the remaining adjacent bend points are then connectedwith a straight line. However, not all small bend angle points can beeliminated. The present system thus discriminates in identifying smallbend angle points that cannot be eliminated where eliminating theparticular point would:

1) pull the rod away from any of the screws by an amount exceeding thevalues ImplantTolerancesXY or ImplantTolerancesXZ. This determination ismade using the ErrorXZ and ErrorXY equations above using the closestremaining bend points (x₁, y₁, z₁) and (x₂, y₂, z₂) on either side ofthe screw position (x_(screw), y_(screw), z_(screw));

2) cause any of the bend angles to exceed the maximum desired bendangle. A bend angle is determined by the arc-cosine of the dot productof the vectors V12 and V23 formed between adjacent bend points (x₁, y₁,z₁)−(x₂, y₂, z₂) and (x₂, y₂, z₂)−(x₃, y₃, z₃).

3) cause the rod to screw interaction to exceed that allowed by theMaxHeadAngle value, calculated using the equation set forth above forcalculating Rod2ScrewAngle.

It can be appreciated that for the “simplest” bend case, the maximumpermitted bend angle may be larger than for the “smoothest” bend case.Conversely, the smoother bend case will necessarily include moreintervening bend points along the length of the rod.

An exemplary bend reduction process is depicted in the sequence of FIGS.15 a-15 j . The XZ and XY plane calculated rod contour is illustrated inFIG. 15 a with bends every ½ cm. In FIG. 15 b , one bend point at the9.5 cm location has been eliminated. In FIG. 15 c , the immediatelyadjacent bend point at location 9.0 mm has been eliminated. It can beeasily appreciated that the elimination of these two bend points doesnot significantly alter the overall contour of the rod. In FIG. 15 d ,the bend at location 8.5 cm has also been eliminated, again with nosignificant impact on the overall contour.

As also shown in FIG. 15 d , the bend at location 1.5 cm has beeneliminated. In successive steps, bends at points 2.0, 2.5, 3.0 and 3.5are eliminated and replaced with straight line segments, as reflected inFIG. 15 e . As the process continues, additional bend points areeliminated and replaced by straight line segments between the remainingadjacent points. Thus, the present system is operable to producemodified rod bend contours shown in FIGS. 15 f-15 j . A comparisonbetween the bend map shown in FIG. 15 a and that shown in FIG. 15 jreveals that the number of bends has been significantly reduced—from 32bends to 7 bends. While every nuance of the calculated contour is notpresent in the final reduced configuration, the overall shape of the rodfollows the calculated design and is certainly sufficiently close to theoptimum design to easily mate with the implanted screws.

As explained above, the process of reducing the number of bends is basedin part on the maximum desired bend angle. In the final version shown inFIG. 15 j , the maximum bend angle was 38 degrees. For a smaller maximumbend angle, 22 degrees, the contour will require a greater number ofbends (12).

It can also be appreciated that the present system generates the seriesof bend point modifications depicted in the sequence of FIGS. 15 a-15 j. If the surgeon selects the simplest bend, the system will output benddata corresponding to FIG. 15 j . If the surgeon selects the smoothestbend type, the output data will correspond to the initial bend curveshown in FIG. 15 a . However, the surgeon may make the bend typeselection on a continuum incorporating aspects of both simple and smoothbends. More particularly, any one of the modified bend configurations inFIGS. 15 b-15 i may be selected as corresponding to a ratio of simpleand smooth, as would a host of other bend point location options.Ultimately, the size of the maximum permissible bend angle chosen willcause the elimination of certain bend locations and not others, withgreater number of bends associated with smaller permissible maximum bendangles and overall smoother resultant shaped outputs.

In accordance with one embodiment of the invention, a GUI is providedfor the surgeon to input data and make selections to produce bend data.It is understood that the surgical objects to be achieved by the bentlinking device or rod may determine the eventual nature of the benddata. Such surgical objectives include to address, straighten, or alterabnormalities in alignment of the body part(s) of the patient; create,lessen or eliminate deformities; reduce or impose changes in alignment;or the addition or elimination of stresses.

The GUI in one specific embodiment is illustrated in FIG. 16 . The GUImay incorporate pull-down menus for entry of case-specific informationsuch as rod type and size, case type, implant system, the range ofinstrumented levels and the amount of overhang of the rod beyond theupper and lower levels. The GUI may also include a sliding scale forselection of bent type, as discussed above. A message panel indicatesthe action to be taken on the GUI, such as “Press the ‘Start’ button tobegin”, identifying the orientation of the screw location data duringdigitizing and calculating the bend profile.

The x, y, z location for the implanted screws is input through the GUI,resulting in the screw location data shown In the data box adjacent the“Get Point” button shown in FIG. 17 . This screw location informationmay be obtained in a conventional manner, as described above, such asusing known 30 digitizers. In some cases, it is difficult to obtainaccurate data with the stylus provided with typical 3D digitizersystems. It is especially difficult to obtain accurate indications ofthe angle of the head of the fastener to which the rod or plate is to beengaged after contouring. Accordingly, one aspect of the presentinvention contemplates a digitizer probe that can be integrated with thedigitizer instrument of these prior systems. In one embodiment, thedigitizer probe 200, shown in FIG. 20 , includes an elongated body 201terminating in a top 202. The body preferably tapers along portion 206to the tip 202. The proximal portion of the body is in the form of ashaft 204 that is configured at its proximal end 205 to mate with thedigitizer instrument. Alternatively, the probe 200 can be formed as anintegral part of the digitizer instrument.

The probe 200 is configured to mate with the head 192 of a fastener,such as the poly-axial fastener 190 shown in FIG. 20 , or head 292 ofthe fastener 290 shown in FIG. 23 . The fastener head includes a toolrecess 194 that is configured to engage a driving tool. In a typicalfastener, the recess is configured as a hex socket or a TORX socket. Thetip 202 of the probe is configured to fit snugly within the recess 194.In one embodiment, shown in FIG. 21 , the tip 202 is circular incross-section with the radius of the tip slightly smaller than the flatdimension of the recess 194. In an alternative embodiment, the tip 202′may be complementary configured with the recess, as shown in FIG. 22 .In this embodiment the tip 202′ has a hex configuration to mate with thehex socket 194. The probe 200 with the tip 202′ may be disengaged fromthe digitizer instrument once the fastener locations have beendetermined and engaged to a driving tool.

The tip 202 has a length sufficient to be fully seated within the recess194 (or the recess 294 of the screw 290 shown in FIG. 23 ). Thisinterface helps ensure that the probe 200 is aligned with the fastener190 so that the angular orientation of the fastener can be accuratelydetermined. In some cases the fastener includes a yoke 195 for apoly-axial connection to the fixation rod. The arms 196 of the yoke forma U-shape to receive the fixation rod. The arms 196 may also provide aguide for alignment of the probe 200, particularly by contact with thetapered portion 206. The tapered portion thus ensures that the probe isin stable engagement with the fastener 194 even when the tip 202 is notfully seated within the recess 194.

A probe 210 is shown in FIG. 24 that is configured to engage thepoly-axial fastener 190. The probe 210 includes a body 211 defining acentral hub 212 and outer wings 214. The body further includes a shaft216 that is sized and configured to integrate with the digitizerinstrument. The hub and wings are configured to be juxtaposed with theopposite faces of the arms 196 of the yoke 195. The central hub 212 canbe configured as a generally rectangular body that extends along theU-shaped opening of the yoke. Alternatively, where the yoke 195 definesa cylindrical cavity between the arms 196, such as to engage a setscrew, the central hub 212 may be circular in cross-section to mate withthe cavity. Likewise, the wings 214 are configured complementary to theouter surface of the arms 196 of the yoke. In a typical case, the armsof the yoke have a cylindrical outer surface, so the interior surface ofthe wings 214 are similarly cylindrical. This configuration allows theprobe 210 to be used as a tool to re-orient or rotate the yoke 195relative to the fastener 190.

The distal end 213 of the central hub 212 may be configured to engagethe upper surface of the head 192 of the bone screw. Alternatively, thehub and wings may define a perimeter channel 218 that is configured tocontact the top of the arms 196 of the yoke 195. In either case, thedistal end 213 or the channel 218 stabilize the probe 210 when itengages the fastener 190 to ensure an accurate angular orientation ofthe probe. It can be appreciated that in this embodiment, the probe 210may be keyed off the position and orientation of the yoke 195, ratherthan the screw head 192. In this case, the distal end 213 of the centralhub is sized to provide clearance from the upper surface of the head.

It is contemplated that the probes 200 and 210 can be provided inconfigurations for mating with specific fastener types. Moreover, thelength of the probe from the tip 202 to the proximal end of the shaft206 of the probe 200 (or from the distal end 213 or channel 218 to theend of the shaft 216 in the embodiment of FIG. 24 ) is precisely known.This length can be calibrated into the digitizing routine to yieldaccurate data about the fastener position in six degrees of freedom,including the angular orientation of the mating features of thefastener. It is further contemplated that the probe 200/210 itself maybe used to identify the angular orientation of the attachment elementrelative to the spine. In this approach, the digitizing instrument cancontact the probe at its proximal end and at a known position adjacentthe interface of the probe with the attachment element. Thethree-dimensional positional data for these two points can then be usedto calculate the spatial angle of the attachment element. This spatialangle can be used particularly to determine wither the yoke 195 ofcertain attachment elements are properly oriented to accept a linkingelement, such as a spinal rod.

The probes 200,210 may be formed of any biocompatible material that issufficiently rigid to resist bending during the digitization process.Where the probe incorporates a “tool” feature, such as the tipconfiguration shown in FIG. 22 , the probe must be able to transmitsufficient torque to the fastener.

Returning to FIG. 16 et seq., the present invention contemplates that asurgeon may desire to achieve a predetermined deformity correction.However, the digitized data corresponds to the actual position of themating elements of the fasteners. In certain cases, this data isdesirable since the object is to mate a bent rod to engage the fastenersin those very positions. However, in some cases, a surgeon may find itdesirable to impart a predetermined correction to the existing curvatureof the spine. For instance, in the case of a scoliosis condition it maybe desirable to shift certain vertebrae in the transverse direction toreduce the scoliotic curvature. The GUI of the present system allows thesurgeon to modify the fastener position data form the original digitizedpositions. In the subsequent steps of the procedure, the bentconfiguration of the spinal rod is determined and the surgeon canevaluate the resultant predicted curvature or shape to determine whetherthe desired correction has been obtained. If necessary the surgeon canrepeat the initial step of establishing the fastener location and adjustthe amount of modification to achieve the desired resultant shape.

After all of the screw location data has been entered, the systemcalculates the bend data based upon the algorithms described above andthe surgeon's selection of bend type. The output on the GUI is asequence of bend data, as shown in FIG. 17 . In the illustratedembodiment, the bend data is tailored to the bending tool 70 describedherein. The magnitude of the bend in this embodiment is represented byletters, in this case “F” thru “I”, that correspond to specific bendangles that are predefined on the bending tool 70. For example, an “I”bend is greater than an “H” bend, and so on.

Once the bend points are established, the present system translates thebend point data into the instructions for the bending tool. As indicatedabove, for the tool 70 described herein, only three data points arenecessary—all derived from the distance from the last bend, the rotationof the rod compared to the bend angle of the last bend and the amount ofthe bend. These values can be obtained from the relationships describedbelow.

The distance between bends is given by the expression((x₂−x₁)²+(y₂−y₁)²+(z₂−z₂))²)^(1/2). The location of the bends is acumulative summation of the distances between bend points.

The rotation between bends can be determined by the angle between thenormal to planes containing successive bend points. For instance, forthe rotation between bend 2 and bend 3, a determination is made of theangle between the normal to the plane N₁₂₃ containing the three bendpoints x₁y₁z₁, x₂y₂z₂, and x₃y₃z₃ and the plane N₂₃₄ containing thethree bend points x₂y₂z₂, x₃y₃z₃, and x₄y₄z₄. The rotation between thesebends is then represented by the arc-cosine of the dot product of N₁₂₃and N₂₃₄.

The amount of the bend is the angle between the vectors containing thebend points. Thus for the example with bend 2 and bend 3, the amount ofthe bend is given by the arc-cosine of the dot product of the vectorsV₁₂ and V₂₃.

It is contemplated that the amount of the bend at each bend point may bealtered to account for springback of the material. For a typical casethe springback will be derived from a linear function, approximated as14 degrees for a 5.5 mm stainless steel rod, or 13 degrees for a 5.5 mmtitanium rod, based on the elasticity of the two materials.

The manner in which the bend data is implemented using the bending tool70 is depicted in FIGS. 18 a-d . In FIG. 18 a , the first bend is madeusing the tool. With the rod 10 held in place by the collet 75 {see FIG.7 ), the slide block 76 is moved to the axial location “24” along thehandle 72 identified in the bend data. In addition to the click stops 77described above, the handle 72 may also incorporate numerical indicia 77a that corresponds to the axial position number in the bend data shownon the GUI.

The bend rotation value of “300” in the bend data is implemented byrotating the collet knob 90 to the appropriate indicia 90 a. Rotatingthe collet knob rotates the rod 10 relative to the bending dies 81, 82,as described in more detail above. Finally, the bend magnitude or anglecorresponding to the value “I” in the bend data, is set using the anglegauge 85. In addition to the ratchet teeth 86 used to establish the 5degree angle increments, the angle gauge 85 may incorporate indicia 86 acorresponding to the bend values “F”-“I” in the bend data of the presentillustration. The gauge may include many more incremental bend angleindicia, ranging from “A” to “N” in the embodiment illustrated in FIG.18 a , thereby providing 14 discrete bend angles. In another embodiment,nondiscrete or continuous bend angles could be employed as could eithersmaller or larger steps between angle choices. Once the components ofthe bending tool have been set according to the calculated bend data,the bend is made, as shown in the figure.

The second bend is accomplished as shown in FIG. 18 b . In this case,the slide block 76 is advanced to the axial location “35”, the colletknob 90 is rotated to the 20 degree position, and the bend angle “H” isselected on the angle gauge 85. The second bend is then made. The effectof the third and fourth bends are shown in FIG. 18 c , with theunderstanding that the bending tool 70 is manipulated according to thebend data, as described above. The final bend is made as shown in FIG.18 d , resulting in a rod 10 that is bent to follow a three-dimensionalcontour calculated to mate with an array of screws implanted in apatient's spine, as shown in FIG. 19 .

In some procedures the spine is instrumented with multiple linkingdevices. For instance, attachment elements and linking elongated rodsmay be positioned on either side of the spinous processes. The two rodsare typically interconnected using transverse connectors to provide arigid “scaffold” for supporting the spine. The method described abovecan be used to generate appropriately shaped rods to be positioned oneither side of the spinal mid line. The bend curves calculated for eachrod may be used to determine the size of any transverse connectors orlinking devices that may be utilized.

The above examples and particular embodiment are not intended to limitthe claims which follow. A variety of changes to the gauges, levers andthe device and method of determining the shaping parameters is withinthe scope of the present invention.

What is claimed is:
 1. A method comprising: determining a plurality oflocations relative to a patient's spine from one or more images of thepatient's spine; generating a bend curve based on the plurality oflocations; selecting a spinal rod; generating bending instructions forbends to be performed on the spinal rod by a bending tool to achieve thebend curve; forming a bent spinal rod, wherein the forming includesbending the spinal rod based on the bending instructions; and providingthe bent spinal rod for implantation in the patient.
 2. The method ofclaim 1, wherein the determining occurs away from the patient and isperformed at least partially manually.
 3. The method of claim 1, whereinthe selecting of the spinal rod is based on the bend curve.
 4. Themethod of claim 1, wherein at least one of the plurality of locationscorresponds to a desired location of a pedicle screw in the patient'sspine.
 5. The method of claim 1, further comprising implanting the bentspinal rod in the patient, wherein the implanting includes coupling thebent spinal rod to a plurality of pedicle screws implanted in thepatient's spine.
 6. The method of claim 5, further comprising implantingthe plurality of pedicle screws.
 7. The method of claim 1, wherein theone or more images of the patient's spine are preoperative radiographs.8. The method of claim 1, further comprising: determining a correctionto impart to an existing curvature of the patient's spine, and whereinthe bend curve is based on the determined correction.
 9. The method ofclaim 1, wherein at least one of the plurality of locations correspondto locations of attachment elements implanted in the patient's spine.10. A method for shaping a spinal rod comprising: obtaining data for aplurality of locations relative to a patient's spine; generating a bendcurve having a plurality of bends, wherein the bend curve is based onthe data for plurality of locations relative to a patient's spine;generating bending instructions for bends to be performed on a spinalrod by a bending tool to achieve the plurality of bends; forming a bentspinal rod, wherein the forming includes bending the spinal rod based onthe bending instructions; and providing the bent spinal rod forimplantation in the patient.
 11. The method of claim 10, furthercomprising at least partially manually determining the plurality oflocations relative to the patient's spine.
 12. The method of claim 10,further comprising at least partially manually determining the pluralityof locations relative to the patient's spine using at least one X-rayimage.
 13. The method of claim 10, further comprising implanting thebent spinal rod in the patient.
 14. The method of claim 13, wherein theimplanting includes coupling the bent spinal rod to a plurality ofpedicle screws implanted in the patient's spine.
 15. The method of claim10, further comprising: determining a correction to impart to anexisting curvature of the patient's spine; and wherein the bend curve isbased on the determined correction.
 16. The method of claim 10, whereineach of the plurality of locations correspond to locations forattachment elements implantable in the patient's spine.
 17. The methodof claim 10, further comprising: deferring implanting the bent spinalrod.
 18. The method of claim 10, wherein the forming of the bent spinalrod occurs using an automated device.
 19. A method comprising:determining, away from the patient, a plurality of locations relative toa patient's spine from one or more preoperative radiographs of thepatient's spine; determining a correction to impart to an existingcurvature of the patient's spine; generating a bend curve based on theplurality of locations, wherein the generating is further based on thedetermined correction; selecting a spinal rod; generating bendinginstructions for bends to be performed on the spinal rod by a bendingtool to achieve the bend curve; forming a bent spinal rod, wherein theforming includes bending the spinal rod based on the bendinginstructions; and providing the bent spinal rod for implantation in thepatient.
 20. The method of claim 19, further comprising: implanting aplurality of pedicle screws in the patient; and implanting the bentspinal rod in the patient, wherein implanting the bent spinal rod in thepatient includes coupling the bent spinal rod to the plurality ofpedicle screws.